Inkjet ink and inkjet recording method

ABSTRACT

The present invention relates to a kind of ink containing at least a pigment, first to third water-soluble solvent groups and water, wherein the first water-soluble solvent group contains one or more kind(s) water-soluble solvent having a solubility parameter SP 1 , the second water-soluble solvent group contains one or more kind(s) of water-soluble solvents having a solubility parameter which is at least 1 greater than SP 1 , and the third water-soluble solvent group contains one or more kind(s) of water-soluble solvents having a solubility parameter which is at least 1 less than SP 1 .

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority under 35USC 119 from JapanesePatent Application No. 2002-359184, the disclosure of which isincorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to inkjet ink and an inkjetrecording method.

[0004] 2. Description of the Related Art

[0005] So-called inkjet systems in which a liquid or melt solid ink isdischarged through an ink discharge port such as a nozzle, a slit and aporous film are used in many printers due to their characteristics suchas small size and low cost. Among these inkjet systems, a piezo-inkjetsystem for discharging ink utilizing deformation of a piezoelectricelement and a thermal inkjet system utilizing the boiling of ink causedby the thermal energy are frequently utilized from the viewpoint of thehigh resolution and the high speed printability. In addition, an inkjetprinter can print not only on a so-called paper such as a plain paper, apaper exclusively for inkjet but also on a film such as an OHP sheet, acloth and the like.

[0006] For the purpose of the long-term storage stability, there isproposed use of a solvent represented by the following formula (II) asan organic solvent (for example, see Japanese Patent ApplicationLaid-Open (JP-A) No. 05-17714). In order to secure the long-term storagestability, it is necessary to prevent water from evaporating. In theprior art, from such a viewpoint, a solvent represented by the generalformula (II) has been used as a humectant. However, in a case of a kindof ink in which a pigment and a solvent represented by the generalformula (II) are combined, reduction in the storage stability associatedwith aggregation of a pigment occurs in some cases.

HO—(CHR—CH₂—O—)_(n)—H  General formula (II)

[0007] [In the general formula (II), n represents an integer, and Rrepresents hydrogen or an alkyl group.]

[0008] In addition, for the purpose of improving the long-term storagestability, the drying property in a plain paper, the printing quality,the water resistance and the abrasion resistance, there is provided akind of ink for inkjet recording containing water, a pigment which isself-dispersible in water and a water-soluble organic solvent, andcontaining a water-soluble organic compound having the S.P. value of 12or smaller and the surface tension at 25° C less than 40 mN/m in anamount of 3.0% by mass to 15.0% by mass relative to a total mass of ink(JP-A No. 11-228898). Example of the same reference shows a kind of inkin which triethylene glycol is added as another additive, but it wasfound that when such the ink is stored for a longer period of time, thestorage stability is insufficient.

[0009] In addition, there is provided an image forming method of usingan ink composition containing a coloring material having at least onecarboxyl group, and a multivalent metal salt solution, adding themultivalent metal salt solution to a recording material and, thereafter,printing using the ink composition (for example, see JP-A No.05-202328). An object of this method is to improve the water resistanceand color bleeding, but when a pigment is used as a coloring material,the long-term ejection stability is reduced in some cases.

[0010] As described above, according to the conventional methods, thelong-term ejection stability, the long-term storage stability, theoptical concentration, the bleeding and the water resistance can not besatisfied at the same time.

SUMMARY OF THE INVENTION

[0011] An object of the present invention is to solve the aforementionedproblems. That is, an object of the invention is to provide a kind ofinkjet ink and an inkjet recording method excellent in the long-termejection stability, the long-term storage stability, the opticalconcentration, the bleeding and the water resistance.

[0012] In order to solve the problem of the invention, the presentinventors intensively studied and, as a result, devised the inventionexplained below, which can satisfy the long-term ejection stability, thelong-term storage stability, the optical concentration, the bleeding andthe water resistance at the same time.

[0013] The first aspect of the invention is to provide an inkjet ink (J)comprising at least a pigment, a water-soluble solvent and water, thewater-soluble solvent comprising:

[0014] a first water-soluble solvent group containing at least one kindof water-soluble solvent represented by the following general formula(I);

[0015] a second water-soluble solvent group containing at least one kindof water-soluble solvent having a solubility parameter which is at least1 greater than a solubility parameter SP₁ of the at least one kind ofwater-soluble solvent represented by general formula (I), and;

[0016] a third water-soluble solvent group containing at least one kindof water-soluble solvent having a solubility parameter which is at least1 less than the solubility parameter SP₁;

[0017] wherein respective contents (% by mass) of the water-solublesolvent groups contained in the inkjet ink satisfy the followingequation (1) and the following equation (2):

HO—(CHR—CH₂—O—)_(n)—H  General formula (I)

W ₂ /W ₁≧1.0  Equation (1)

0.25≦W ₃ /W ₁≦0.75  Equation (2)

[0018] wherein in general formula (I), equation (1) and equation (2), nrepresents an integer of 3 to 6; R represents hydrogen or a methylgroup; W₁ represents a content (% by mass) of the first water-solublesolvent group contained in the inkjet ink; W₂ represents a content (% bymass) of the second water-soluble solvent group contained in the inkjetink; and W₃ represent a content (% by mass) of the third water-solublesolvent group contained in the inkjet ink.

[0019] The second aspect of the invention is to provide an inkjet ink(J), wherein the first water-soluble organic solvent group furthercontains a water-soluble solvent other than the at least one kind ofwater-soluble solvent represented by general formula (I), having asolubility parameter such that an absolute value of a difference thereoffrom the solubility parameter SP₁ is less than 1.

[0020] The third aspect of the invention is to provide an inkjet ink(J), wherein the pigment is self-dispersible in water.

[0021] The fourth aspect of the invention is to provide an inkjet ink(J), further containing a polymer anion or a polymer cation, wherein thepigment is self-dispersible in water.

[0022] The fifth aspect of the invention is to provide an inkjet ink(J), further comprising a polymer dispersant, wherein the pigment isdispersed by the polymer dispersant.

[0023] The sixth aspect of the invention is to provide an inkjet ink(J), wherein the first water-soluble organic solvent group furthercontains a water-soluble solvent other than the at least one kind ofwater-soluble solvent represented by general formula (I), having asolubility parameter such that an absolute value of a difference thereoffrom the solubility parameter SP₁ is less than 1, and an absolute valueof a ζ potential of the inkjet ink is in the range of 3 mV to 60 mV.

[0024] The seventh aspect of the invention is to provide an inkjet ink(J), wherein a surface tension of the inkjet ink is at least 20 Nm/m andless than 45 mN/m.

[0025] The eighth aspect of the invention is to provide an inkjet ink(J), wherein a viscosity of the inkjet ink is at least 1.2 mPa·s andless than 6.0 mPa·s.

[0026] The nineth aspect of the invention is to provide an inkjetrecording method (K) of printing on a recording medium containing amultivalent metal salt, an organic cationic substance or an organicanionic substance, using an inkjet ink containing at least a pigment, awater-soluble solvent and water, wherein:

[0027] the water-soluble solvent contains,

[0028] a first water-soluble solvent group containing at least one kindof water-soluble solvent represented by the following general formula(I),

[0029] a second water-soluble solvent group containing at least one kindof water-soluble solvent having a solubility parameter which is at least1 greater than a solubility parameter SP₁ of the at least one kind ofwater-soluble solvent represented by general formula (I), and

[0030] a third water-soluble solvent group containing at least one kindof water-soluble solvent having a solubility parameter which is at least1 less than the solubility parameter SP₁; and

[0031] respective contents (% by mass) of the water-soluble solventgroups contained in the inkjet ink satisfy the following equation (1)and the following equation (2):

HO—(CHR—CH₂—O—)_(n)—H  General formula (I)

W ₂ /W≧1.0  Equation (1)

0.25≦W ₃ /W ₁≦0.75  Equation (2)

[0032] wherein in general formula (I), equation (1) and equation (2), nrepresents an integer of 3 to 6; R represents hydrogen or a methylgroup; W₁ represents a content (% by mass) of the first water-solublesolvent group contained in the inkjet ink; W₂ represents a content (% bymass) of the second water-soluble solvent group contained in the inkjetink; and W₃ represents a content (% by mass) of the third water-solublesolvent group contained in the inkjet ink.

[0033] The tenth aspect of the invention is to provide an inkjetrecording method (K), wherein the first water-soluble organic solventgroup further contains a water-soluble solvent other than the at leastone kind of water-soluble solvent represented by the general formula(I), having a solubility parameter such that an absolute value of adifference thereof from the solubility parameter SP₁ is less than 1.

[0034] The eleventh aspect of the invention is to provide an inkjetrecording method (K), wherein a number of particles having particlediameters of 5 μm or larger contained in the inkjet ink which is addeddropwise to a surface of the recording medium is 1×10²/μl or more.

[0035] The twelfth aspect of the invention is to provide an inkjetrecording method (L) in which a liquid composition containing amultivalent metal salt, an organic cationic substance or an organicanionic substance is imparted to a surface of a recording medium and,thereafter, printing is conducted on a region of the surface of therecording medium to which the liquid composition has been imparted,using an inkjet ink containing at least a pigment, a water-solublesolvent and water, wherein:

[0036] the water-soluble solvent contains

[0037] a first water-soluble solvent group containing at least one kindof water-soluble solvent represented by the following general formula(I),

[0038] a second water-soluble solvent group containing at least one kindof water-soluble solvent having a solubility parameter which is at least1 greater than a solubility parameter SP₁ of the at least one kind ofwater-soluble solvent represented by general formula (I), and

[0039] a third water-soluble solvent group containing at least one kindof water-soluble solvent having a solubility parameter which is at least1 less than the solubility parameter SP₁; and

[0040] respective contents (% by mass) of the water-soluble solventgroups contained in the inkjet ink satisfy the following equation (1)and the following equation (2):

HO—(CHR—CH₂—O—)_(n)—H  General formula (I)

W ₂ /W ₁≧1.0  Equation (1)

0.25≦W ₃ /W ₁≦0.75  Equation (2)

[0041] wherein in general formula (I), equation (1) and equation (2), nrepresents an integer of 3 to 6; R represents hydrogen or a methylgroup; W₁ represents a content (% by mass) of the first water-solublesolvent group contained in the inkjet ink; W₂ represents a content (% bymass) of the second water-soluble solvent group contained in the inkjetink; and W₃ represents a content (% by mass) of the third water-solublesolvent group contained in the inkjet ink.

[0042] The thirteenth aspect of the invention is to provide an inkjetrecording method (L), wherein the first water-soluble organic solventgroup further contains a water-soluble solvent other than the at leastone kind of water-soluble solvent represented by the general formula(I), having a solubility parameter such that an absolute value of adifference thereof from the solubility parameter SP₁ is less than 1.

[0043] The fourteenth aspect of the invention is to provide an inkjetrecording method (L), wherein a number of particles having particlediameters of 5 μm or larger in a mixed solution of the inkjet ink andthe liquid composition is 1.0×10³/μl or more.

[0044] The fifteenth aspect of the invention is to provide an inkjetrecording method (M) of printing on a recording medium by a thermalinkjet system or a piezo-inkjet system using an inkjet ink containing atleast a pigment, a water-soluble solvent and water, wherein:

[0045] the water-soluble solvent contains

[0046] a first water-soluble solvent group containing at least one kindof water-soluble solvent represented by the following general formula(I),

[0047] a second water-soluble solvent group containing at least one kindof water-soluble solvent having a solubility parameter which is at least1 greater than a solubility parameter SP₁ of the at least one kind ofwater-soluble solvent represented by general formula (I), and

[0048] a third water-soluble solvent group containing at least one kindof water-soluble solvent having a solubility parameter which is at least1 less than the solubility parameter SP₁; and

[0049] respective contents (% by mass) of the water-soluble solventgroups contained in the inkjet ink satisfy the following equation (1)and the following equation (2):

HO—(CHR—CH₂—O—)_(n)—H  General formula (I)

W ₂ /W ₁≧1.0  Equation (1)

0.25≦W ₃ /W ₁≦0.75  Equation (2)

[0050] wherein in general formula (I), equation (1) and equation (2), nrepresents an integer of 3 to 6; R represents hydrogen or a methylgroup; W₁ represents a content (% by mass) of the first water-solublesolvent group contained in the inkjet ink; W₂ represents a content (% bymass) of the second water-soluble solvent group contained in the inkjetink; and W₃ represents a content (% by mass) of the third water-solublesolvent group contained in the inkjet ink.

[0051] The sixteenth aspect of the invention is to provide an inkjetrecording method (M), wherein the first water-soluble organic solventgroup contains a water-soluble solvent other than the at least one kindof water-soluble solvent represented by general formula (I), having asolubility parameter such that an absolute value of a difference thereoffrom the solubility parameter SP₁ is less than 1.

[0052] The seventeenth aspect of the invention is to provide an inkjetrecording method (M), wherein an amount of the inkjet ink to be impartedto a surface of the recording medium is 25 ng or less per one droplet.

DETAILED DESCRIPTION OF THE INVSENTION

[0053] The present invention will be explained separately andsequentially for inkjet ink and an inkjet recording method.

[0054] (Inkjet Ink)

[0055] The inkjet ink of the invention (hereinafter, abbreviated as“ink” in some cases) is a kind of inkjet ink containing at least apigment, a water-soluble solvent and water, characterized in that thewater-soluble solvent contains a first water-soluble solvent groupcontaining at least one kind water-soluble solvents represented by thefollowing general formula (I), a second water-soluble solvent groupcontaining one or more kind(s) of water-soluble solvents having asolubility parameter which is at least 1 greater than a solubilityparameter SP₁ of a water-soluble solvent represented by the generalformula (I), and a third water-soluble solvent group containing one ormore kind(s) of water-soluble solvents having a solubility parameterwhich is at least 1 less than the solubility parameter SP₁, and thecontent (% by mass) of each water-soluble solvent group contained in theinkjet ink satisfies the relationship of the following equation (1) andthe following equation (2).

HO—(CHR—CH₂—O—)_(n)—H  General formula (I)

W ₂ /W ₁≧1.0  Equation (1)

0.25≦W ₃ /W ₁≦0.75  Equation (2)

[0056] In the general formula (I), the equation (1) and the equation(2), n represents an integer of 3 to 6, R represents hydrogen or amethyl group, W₁ represents the content (% by mass) of a firstwater-soluble solvent group contained in the inkjet ink, W₂ representsthe content (% by mass) of a second water-soluble solvent groupcontained in the inkjet ink, and W₃ represents the content (% by mass)of a third water-soluble solvent group contained in the inkjet ink.

[0057] When a first water-soluble solvent group contains two or morekinds of water-soluble solvents represented by the general formula (I),a solubility parameter SP₁ of the first water-soluble solvent groupmeans a value obtained by calculating solubility parameters ofwater-soluble solvents represented by the respective formulae (I),taking an average weighted by the content (% by mass) intoconsideration, as shown by the following equation (3). $\begin{matrix}{{SP}_{1} = \frac{\sum\limits^{\quad}\quad \left( {{{SP}(n)} \times {W(n)}} \right)}{\sum{W(n)}}} & {{Equation}\quad (3)}\end{matrix}$

[0058] In the equation (3), SP(n) represents a solubility parameter of an^(th) water-soluble solvent represented by the general formula (I),W(n) represents the content (% by mass) of a n^(th) water-solublesolvent represented by the general formula (I), and n represents aninteger of 1 or larger.

[0059] Since the ink of the invention has the above-explained featuresand satisfies the equations (1) and (2) at the same time, it isexcellent in the long-term ejection stability, the long-term storagestability, the optical concentration, the bleeding and the waterresistance. When three or more kinds of water-soluble solvents containedin ink can not satisfy any of the equation (1) and the equation (2), allof the long-term ejection stability, the long-term storage stability,the optical concentration, the bleeding and the water resistance can notbe satisfied simultaneously at a high level.

[0060] The mechanism of exerting such the effect has not been madeclear, but the present inventor presume as follows:

[0061] First, the present inventors intensively have studied awater-soluble solvent represented by the general formula (I) used in theink of the invention and found that, when this water-soluble solvent isadded to the ink, assuming a combination with a pigment, Drop speed isimproved and Kogation (phenomenon in which as the number of pulsesapplied to a printing head increases, the ink components are adhered ona heater provided on the printing head when printing is performed usinga thermal inkjet system) is prevented.

[0062] On the other hand, one cause of Kogation is contemplated to bedenaturation of ink components by the thermal stress of a heater. Thereis a tendency that, by occurrence of this Kogation, the dischargingproperty of ink is reduced, the optical concentration is reduced, andthe ejection directing property is deteriorated and, as a result, thelong-term ejection stability is reduced.

[0063] However, from the above results, the present inventors presumethat addition of a water-soluble solvent represented by the generalformula (I) to ink increases Drop speed and improves the ejectionproperty of the ink, whereby, deterioration of the long-term ejectionstability caused by Kogation is prevented. In addition, such theprevention of deterioration of the long-term ejection stability ispresumed to be due to some influence of a water-soluble solventrepresented by the general formula (I) on the occurrence of air bubblesupon heating of ink with a heater.

[0064] Although a water-soluble solvent represented by the generalformula (I) has been used also in the conventional ink, such thewater-soluble solvent has been utilized as a humectant for preventingwater in ink from evaporating to improve the storage stability. However,the present inventors intensively have studied and found that, even whenthe water-soluble solvent represented by the general formula (I) isadded to ink, the storage stability is reduced in some cases when theink is stored over a longer period of time. Reduction in such thelong-term storage stability is presumed to be caused by that thewater-soluble solvent represented by the formula (I) interacts with acoloring material to destabilize dispersion of this coloring material.Such presumption is supported by an increase in a particle diameter of acoloring material contained in ink during storage.

[0065] The present inventors found that it is effective to use a kind ofink prepared by combining a first water-soluble solvent group containinga water-soluble solvent represented by the general formula (I) withsecond and third water-soluble solvent groups so that thesewater-soluble solvents satisfy the relationship of the aforementionedequations (1) and (2), in order to solve the aforementioned problems.Further, the present inventors found that, in such the case, in additionto being excellent also in the long-term storage stability withoutdeteriorating the long-term ejection stability, the opticalconcentration, the bleeding and the water resistance can be satisfied ata high level.

[0066] Although the mechanism by which the long-term storage stabilityis also excellent without deteriorating the long-term ejection stabilitylike this has not been made clear, the following two points are presumedto contribute thereto: (1) by adding to ink a third water-solublesolvent group which is more hydrophobic than a first water-solublesolvent group, the solubility of the first water-soluble solvent groupin the ink is improved, and interaction between a pigment and the firstwater-soluble solvent group is reduced, and (2) by adding to ink asecond water-soluble solvent group which is more hydrophilic than thefirst water-soluble solvent group, the moisture retention of the ink issecured to suppress excessive evaporation of water.

[0067] As described above, when three kinds of water-soluble solventgroups contained in the ink of the invention can not satisfy any of theaforementioned equations (1) and (2), the ink of the invention can notsatisfy all of the long-term ejection stability, the long-term storagestability, the optical concentration, the bleeding and the resistance toa water simultaneously at high level. In this case, although it isdifficult to say that there is the clear corresponding relationshipbetween the equations (1) and (2) and these properties, there is thefollowing tendency.

[0068] First, when a value of W₂/W₁ is less than 1, there is a tendencythat the long-term storage property is deteriorated by waterevaporation.

[0069] In addition, when a value of W₃/W₁ is less than 0.25, there is atendency that the long-term storage property is deteriorated due toincrease in a particle diameter of coloring material in an ink (presumedto be due to destabilization of the dispersed state and aggregation).Further, when a value of W₃/W₁ exceeds 0.75, there is a tendency thatthe solubility of the first water-soluble solvent group in the ink isenhanced, and the long-term ejection stability is reduced.

[0070] In the equation (1), a value of W₂/W₁ is preferably 1.5 orlarger, further preferably 2 or more. In addition, in the equation (2),a value of W₃/W₁ is preferably 0.3 to 0.7, more preferably 0.4 to 0.6.

[0071] In the invention, a solubility parameter (SP value) means a valuedefined by the following equation (4). The SP value can be obtained byutilizing the chemical composition, the evaporation heat, the refractiveindex, the kauri butanol value, the surface tension and the like, but inthe invention, the SP value of Fedors calculated from chemicalcomposition is used.

[0072] Equation (4) $\begin{matrix}{{SP} = {\sqrt{\frac{\Delta \quad E}{V}} = \sqrt{\frac{\sum\limits_{i}^{\quad}\quad {\Delta \quad {ei}}}{\sum\limits_{i}^{\quad}\quad {\Delta \quad {vi}}}}}} & {{Equation}\quad (4)}\end{matrix}$

[0073] In the equation (4), SP represents a solubility parameter, ΔErepresents the cohesive energy (cal/mol), V represents molar volume(cm³/mol), Δei represents the evaporation energy (cal/atom or group ofatoms) of i^(th) atom or group of atoms, Δvi represents a molar volume(cm³/atom or group of atoms) of i^(th) atom or group of atoms, and irepresents an integer of 1 or larger.

[0074] Customarily the SP value represented by the equation (4) isobtained so that its unit is call^(1/2)/cm^(3/2), and the value isexpressed as a dimensionless value. Additionally, in the invention,since a relative reference of the SP value between two compounds has thesignificance, the value obtained according to the above custom is used,and the value is expressed as a dimensionless value.

[0075] For reference, when the SP value represented by the equation (4)can be converted into SI unit (J^(1/2)/m^(3/2)), by being multiplied by2046.

[0076] The ink of the invention has, as explained above, thecharacteristics in the kinds of water-soluble solvents contained in theink and their solubility parameters and contents, and desirable physicalproperties other than them will be explained below.

[0077] The absolute value of the ζ potential of a coloring material suchas a pigment and the like contained in the ink of the invention ispreferably 3 mV to 60 mV, more preferably 5 mV to 40 mV, furtherpreferably 7.5 mV to 30 mV. When the absolute value of the ζ potentialis less than 5 mV, the long-term storage stability can not be maintainedin some cases. On the other hand, when the absolute value of the ζpotential exceeds 60 mV, the redispersibility of the coloring materialin water becomes high, and the water resistance is deteriorated in somecases.

[0078] The surface tension of the ink of the invention is preferably atleast 20 mN/m and less than 45 mN/m, more preferably at least 25 mN/mand less than 40 mN/m, where further preferably at least 27.5 mN/m andless than 37.5 mN/m.

[0079] When the surface tension of the ink is less than 20 mN/m, the inkpermeates into the recording medium faster, and the color unevenness dueto printing sequence, the optical concentration and the bleeding aredeteriorated in some cases. Conversely, when the tension exceeds 40mN/m, the ink permeates into the recording medium slower, and the inkconcentration at a solid part some times becomes uneven.

[0080] The viscosity of the ink of the invention is preferably at least1.2 mPa·s and less than 6.0 mPa·s, more preferably at least 1.5 mPa·sand less than 4.5 mPa·s, further preferably at least 1.8 mPa·s and lessthan 4.0 mPa·s.

[0081] When the viscosity of the ink is greater than 6.0 mPa·s, thedischarging property of the ink is reduced, and the reliability isreduced in some cases.

[0082] On the other hand, when the viscosity of the ink is less than 1.2mPa·s, the sufficient optical concentration can not be obtained in somecases, which is thought to be caused by the phenomenon that thepermeability of the ink into a recording medium becomes great, and apigment is permeated into the interior of a recording medium.

[0083] Then, components (pigment, water-soluble solvent, otheradditives) of the ink of the invention will be explained in detail.

[0084] As a pigment used in the invention, organic pigments or inorganicpigments may be used, and examples of black pigments include carbonblack pigments such as furnace black, lamp black, acetylene black,channel black and the like. In addition to pigments of black and threeprimary colors, that is cyan, magenta, and yellow, pigments of specificcolors such as red, green, blue, brown, white and the like, metal lusterpigments such as gold, silver and the like, colorless or pale colorextender pigments, plastic pigments and the like may be used. Inaddition, pigments which are newly synthesized for the invention may beused.

[0085] Examples include Raven 7000, Raven 5750, Raven 5250, Raven 5000ULTRA II, Raven 3500, Raven 2000, Raven 1500, Raven 1250, Raven 1200,Raven 1190 ULTRA II, Raven 1170, Raven 1255, Raven 1080, Raven 1060 (allmanufactured by Columbian Chemicals Company), Regal 400R, Regal 330R,Regal 660R, Mogul L, Black Pearls L, Monarch 700, Monarch 800, Monarch880, Monarch 900, Monarch 1000, Monarch 1100, Monarch 1300, Monarch 1400(all manufactured by Cabot Corporation), Color Black FW1, Color BlackFW2, Color Black FW2V, Color Black 18, Color Black FW200, Color BlackS150, Color Black S160, Color Black S170, Printex 35, Printex U, PrintexV, Printex 140U, Printex 140V, Special Black 6, Special Black 5, SpecialBlack 4A, Special Black 4(all manufactured by Degussa), No.25, No. 33,No.40, No.47, No.52, No.900, No.2300, MCF-88, MA600, MA7, MA8, MA100(all manufactured by Mitsubishi Chemical Co., Ltd.) and the like.However examples are not limited to the above.

[0086] Examples of cyan include C.I. Pigment Blue-1, C.I. PigmentBlue-2, C.I. Pigment Blue-3, C.I. Pigment Blue-15, C.I. PigmentBlue-15:1, C.I. Pigment Blue-15:2, C.I. Pigment Blue-15:3, C.I. PigmentBlue-15:4, C.I. Pigment Blue: 16, C.I. Pigment Blue-22, C.I. PigmentBlue-60 and the like. However examples are not limited to the above.

[0087] Examples of magenta include C.I. Pigment Red-5, C.I. PigmentRed-7, C.I. Pigment Red-12, C.I. Pigment Red-48, C.I. Pigment Red-48: 1,C.I. Pigment Red-57, C.I. Pigment Red-112, C.I. Pigment Red-122, C.I.Pigment Red-123, C.I. Pigment Red-146, C.I. Pigment Red-168, C.I.Pigment Red-184, C.I. Pigment Red-202 and the like. However examples arenot limited to the above.

[0088] Examples of yellow include C.I. Pigment Yellow-1, C.I. PigmentYellow-2, C.I. Pigment Yellow-3, C.I. Pigment Yellow-12, C.I. PigmentYellow-13, C.I. Pigment Yellow-14, C.I. Pigment Yellow-16, C.I. PigmentYellow-17, C.I. Pigment Yellow-73, C.I. Pigment Yellow-74, C.I. PigmentYellow-75, C.I. Pigment Yellow-83, C.I. Pigment Yellow-93, C.I. PigmentYellow-95, C.I. Pigment Yellow-97, C.I. Pigment Yellow-98, C.I. PigmentYellow-114, C.I. Pigment Yellow-128, C.I. Pigment Yellow-129, C.I.Pigment Yellow-138, C.I. Pigment Yellow-151, C.I. Pigment Yellow-154 andthe like. However examples are not limited to the above..

[0089] Alternatively, as a pigment used in the invention, a pigmentwhich is self-dispersible in water may be used. A pigment which isself-dispersible in water refers to a pigment which has manysolubilizing groups for water on its surface, and is stably dispersed inwater in the absense of a polymer dispersant.

[0090] Specifically, a pigment which is self-dispersible in water isobtained by subjecting a general pigment to surface modifying treatmentsuch as acid or base treatment, coupling agent treatment, polymer grafttreatment, plasma treatment, oxidation/reduction treatment and the like.

[0091] Alternatively, as a pigment which is self-dispersible in water,in addition to pigments obtained by subjecting the aforementionedpigment to surface modifying treatment, commercially availableself-dispersible pigments such as Cab-o-jet-200, Cab-o-jet-250,Cab-o-jet-260, Cab-o-jet-270, Cab-o-jet-300, IJX-444 and IJX-55manufactured by Cabot Corporation, and Microjet Black CW-1 and MicrojetBlack CW-2 and the like manufactured by Orient Chemical Industries, Ltd.may be used.

[0092] Also when a pigment which is self-dispersible in water is used asa pigment, an ink may contain a polymer anion or a polymer cation.

[0093] The content of a pigment contained in an ink is preferably in therange of 0.5 to 20% by mass, more preferably in the range of 1 to 10% bymass. When the content of a pigment in an ink is less than 0.5% by mass,the sufficient optical concentration is not obtained in some cases. Whenthe content is more than 20% by mass, the ejection property of an inkbecomes unstable in some cases.

[0094] The following polymer compounds may be added to the ink of theinvention. These polymer compounds may be used as a pigment dispersantfor dispersing a normal pigment, or alternatively, they may be used asan additive when a pigment which is self-dispersible in water is used asa pigment.

[0095] As a polymer compound, nonionic compounds, anionic compounds,cationic compounds, amphoteric compounds and the like can be used. Forexample, a copolymer of a monomer having an α, β-ethylenic unsaturatedgroup can be used.

[0096] Examples of a monomer having an α, β-ethylenic unsaturated groupinclude acrylic acid, methacrylic acid, crotonic acid, itaconic acid,itaconic acid monoester, maleic acid, maleic acid monoester, fumaricacid, fumaric acid monoester, vinylsulfonic acid, styrenesulfonic acid,sulfonated vinylnaphthlene, vinylalcohol, acrylamide, methacryloxyethylphosphate, bismethacryloxyethyl phosphate, methacryloxyethylphenylacidphosphate, ethylene glycol dimethacrylate, diethylene glycoldimethacrylate, styrene, styrene derivatives such as a-methylstyrene,vinyltoluene and the like, vinylcyclohexane, vinylnaphthalene,vinylnaphthalene derivatives, acrylic acid alkyl ester, acrylic acidphenyl ester, methacrylic acid alkyl ester, methacrylic acid phenylester, methacrylic acid cycloalkyl ester, crotonic acid alkyl ester,itaconic acid dialkyl ester, maleic acid dialkyl ester and the like.

[0097] A copolymer obtained by copolymerizing one or more kind(s) ofmonomer having an α, β-ethylenic unsaturated group alone or a pluralityof the polymers can be used as a polymer dispersant. Examples thereofinclude polyvinyl alcohol, polyvinylpyrrolidone, styrene-styrenesulfonicacid copolymer, styrene-maleic acid copolymer, styrene-methacrylic acidcopolymer, styrene-acrylic acid copolymer, vinylnaphthalene-maleic acidcopolymer, vinylnaphthalene-methacrylic acid copolymer,vinylnaphthalene-acrylic acid copolymer, acrylic acid alkylester-acrylic acid copolymer, methacrylic acid alkyl ester-methacrylicacid, styrene-methacrylic acid alkyl ester-methacrylic acid copolymer,styrene-acrylic acid alkyl ester-acrylic acid copolymer,styrene-methacrylic acid phenyl ester-methacrylic acid copolymer,styrene-methacrylic acid cyclohexyl ester-methacrylic acid copolymer andthe like.

[0098] It is preferable that the above-exemplified polymer compoundswhich are added to an ink have a mass average molecular weight in therange of 2000 to 15000. When a molecular weight of a polymer substanceis less than 2000, the pigment dipersibility is reduced in some cases.On the other hand, when a molecular weight exceeds 15000, the viscosityof the ink is increased, and the discharging property is deteriorated insome cases. A more preferable mass average molecular weight of a polymercompound is in the range of 3500 to 10000.

[0099] When a polymer compound is added to an ink, it is preferable thatan addition amount is in the range of 0.1 to 3% by mass. When anaddition amount exceeds 3% by mass, the viscosity of the ink isincreased, and the ejection property of the ink becomes unstable in somecases. On the other hand, when an addition amount is less than 0.1% bymass, the dispersion stability of a pigment is decreased in some cases.An amount of a polymer compound to be added to the ink is morepreferably in the range of 0.15 to 2.5% by mass, further preferably inthe range of 0.2 to 2% by mass.

[0100] As a water-soluble organic solvent used in the invention, theknown water-soluble solvents such as polyhydric alcohols, polyhydricalcohol derivatives, nitrogen-containing solvents, alcohols,sulfur-containing solvents and the like can be used.

[0101] Examples of polyhydric alcohols include ethylene glycol,diethylene glycol, propylene glycol, butylene glycol, triethyleneglycol, tetraethylene glycol, pentaethylene glycol, hexaethylene glycol,tripropylene glycol, 1,5-pentanediol, 1,2,6-hexanetriol, glycerin andthe like.

[0102] Examples of polyhydric alcohol derivatives include ethyleneglycol monomethyl ether, ethylene glycol monoethyl ether, ethyleneglycol monobutyl ether, diethylene glycol monomethyl ether, diethyleneglycol monoethyl ether, diethylene glycol monobutyl ether, propyleneglycol monobutyl ether, dipropylene glycol monobutyl ether, ethyleneoxide adduct of diglycerin, and the like.

[0103] Examples of a nitrogen-containing solvent include pyrrolidone,N-methyl-2-pyrrolidone, cyclohexylpyrrolidone, triethanolamine and thelike, examples of alcohols include alcohols such as ethanol, isopropylalcohol, butyl alcohol, benzyl alcohol and the like, and examples ofsulfur-containing solvents include thiodiethanol, thiodiglycerol,sulfolane, dimethyl sulfoxide and the like. Besides, propylenecarbonate, ethylene carbonate, and the like may be used.

[0104] For the first water-soluble solvent group, it is required thatthe water-soluble solvent represented by the general formula (I) isused. Specifically, one kind selected from the group of triethyleneglycol, tetraethylene glycol, pentaethylene glycol, hexaethylene glycoland tripropylene glycol, or a combination of two or more of them can beused.

[0105] When the first water-soluble solvent group contains two or morekinds of water-soluble solvents represented by the general formula (I),a solubility parameter SP₁ of the first water-soluble solvent group iscalculated according to the already described equation (3).

[0106] In addition, the first water-soluble solvent group may containone or more kind(s) of water-soluble solvents other than the generalformula (I). A water-soluble solvent other than the general formula (I)contained in the first water-soluble solvent group (hereinafter,abbreviated as “water-soluble solvent other than the general formula(I)” in some cases) means a water-soluble solvent having a solubilityparameter such that an absolute value of a difference between itssolubility parameter and solubility parameter SP₁ (in the case of two ormore kinds, a value obtained based on the equation (3)) of one or morekind(s) of water-soluble solvents represented by the general formula (I)is less than 1.

[0107] When a water-soluble solvent other than the general formula (I)is contained in the first water-soluble solvent group in addition to onekind water-soluble solvent represented by the general formula (I), acombination of them is not particularly limited as far as the solubilityparameters satisfy the aforementioned conditions, but examples thereofinclude the following combinations.

[0108] When triethylene glycol is used as the water-soluble solventrepresented by the general formula (I), one or more kind(s) of1,2-hexanediol, polyvinylpyrrolidone and ethylene oxide adduct ofdiglycerin can be used by combination as the water-soluble solvent otherthan the general formula (I). When tetraethylene glycol is used as thewater-soluble solvent represented by the general formula (I), one ormore kind(s) of 1,2-hexanediol, ethylene oxide adduct of diglycerin,polyvinylpyrrolidone, 2-pyrrolidone and ethylene glycol monomethyl ethercan be used by combination as the water-soluble solvent other than thegeneral formula (I). When pentaethylene glycol is used as thewater-soluble solvent represented by the general formula (I), one ormore kind(s) of diethylene glycol monomethyl ether, propylene glycolmonomethyl ether, ethylene glycol monoethyl ether, polyvinylpyrrolidone,ethylene oxide adduct of diglycerin, ethylene glycol monomethyl etherand 2-pyrrolidone can be used by combination as the water-solublesolvent other than the general formula (I).

[0109] In addition, when hexaethylene glycol is used as thewater-soluble solvent represented by the general formula (I), one ormore kind(s) of polyvinylpyrrolidone, 2-pyrrolidone, ethylene glycolmonomethyl ether, ethylene oxide adduct of diglycerine, ethylene glycolmonoethyl ether, propylene glycol monomethyl ether, diethylene glycolmonomethyl ether, propylene glycol monomethyl ether and diethyleneglycol monoethyl ether can be used by combination as the water-solublesolvent other than the general formula (I). When tripropylene glycol isused as the water-soluble solvent represented by the general formula(I), one or more kind(s) of diethylene glycol monomethyl ether,propylene glycol monomethyl ether, ethylene glycol monoethyl ether,ethylene oxide adduct of diglycerin, ethylene glycol monomethyl ether,polyvinylpyrrolidone and 2-pyrrolidone can be used by combination as thewater-soluble solvent other than the general formula (I).

[0110] In the case two or more kinds of water-soluble solventsrepresented by the general formula (I) are used, one or more kind(s) ofthe water-soluble solvents other than the general formula (I) (e.g. theabove-exemplified water-soluble solvents other than the general formulae(I)) having a solubility parameter is/are selected, and can be used bycombination, such that an absolute value of a difference between itssolubility parameter and a solubility parameter SP₁ of water-solublesolvents represented by the general formula (I)calculated according tothe already described equation (3) is less than 1.

[0111] In addition, the second water-soluble solvent group is notparticularly limited as far as it is a water-soluble solvent having asolubility parameter which is at least 1 greater than a solubilityparameter SP₁ of the first water-soluble solvent group. For example, itis preferable to select and use solvents satisfying the aforementionedrelationship among the group consisting of ethylene glycol, diethyleneglycol, propylene glycol, glycerin, ethanol and isopropyl alcohol, and aplurality of such solvents may be used.

[0112] Similarly, the third water-soluble solvent group is notparticularly limited as far as it is a water-soluble solvent having asolubility parameter which is at least 1 less than a solubilityparameter SP₁ of the first water-soluble solvent group. For example, itis preferable to select and use solvents satisfying the aforementionedrelationship among the group consisting of ethylene glycol monomethylether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether,diethylene glycol monomethyl ether, diethylene glycol monoethyl ether,diethylene glycol monobutyl ether, propylene glycol monobutyl ether,dipropylene glycol monobutyl ether, ethylene oxide adduct of diglycerin,pyrrolidone, N-methyl-2-pyrrolidone, cyclohexylpyrrolidone andsulfolane, and a plurality of them may be used.

[0113] The content of all water-soluble organic solvents contained inthe ink is preferably in the range of 1 to 60% by mass, more preferablyin the range of 5 to 40% by mass.

[0114] When the content of all water-soluble organic solvents containedin the ink is less than 1% by mass, the sufficient optical concentrationis not obtained in some cases. Conversely, when the content is greaterthan 60% by mass, the viscosity of the ink is increased, and theejection property of the ink becomes unstable in some cases.

[0115] A surfactant may be added to the ink of the invention ifnecessary. As this surfactant, a compound having a structure having bothof a hydrophilic part and a hydrophobic part in a molecule can be used.Any of an anionic surfactant, a cationic surfactant, an amphotericsurfactant and a nonionic surfactant may be used. Alternatively, thealready described polymer compound (polymer dispersant) may be used alsoas a surfactant.

[0116] As the anionic surfactant, alkylbenzenesulfonate,alkylphenylsulfonate, alkylnaphtharene sulfonate, higher fatty acidsalt, sulfate ester salt of higher fatty acid ester, sulfonate salt ofhigher fatty acid ester, sulfate ester salt and sulfonate salt of higheralcohol ether, higher alkyl sulfosuccinate salt, higher alkyl phosphateester salt, phosphate ester salt of ethylene oxide adduct of higheralcohol and the like can be used. For example, dodecylbenzenesulfonate,kerylbenzenesulfonate, isopropylnaphthalenesulfonate,monobutylphenylphenolmonosulfonate, monobutylbiphenylsulfonate,dibutylphenylphenoldisulfonate and the like may be also effectivelyused.

[0117] Examples of the nonionic surfactant include ethylene oxide adductof polypropylene glycol, polyoxyethylene nonylphenylether,polyoxyethylene octyl phenyl ether, polyoxyethylene dodecyl phenylether, polyoxyethylene alkyl ether, polyoxyethylene fatty acid ester,sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester,fatty acid alkylolamide, acetylene glycol, oxyethylene adduct ofacetylene glycol, aliphatic alkanolamide, glycerin ester, sorbitan esterand the like.

[0118] Examples of the cationic surfactant include tetraalkylammoniumsalt, alkylamine salt, benzalkonium salt, alkylpyridium salt,imidazolium salt and the like. More specific examples includedihydroxyethylstearylamine, 2-heptadecenyl-hydroxyethylimidazoline,lauryldimethylbenzylammonium chloride, cetylpyridinium chloride,stearamidomethylpyridinium chloride and the like.

[0119] Besides, silicone type surfactants such as oxyethylene adduct ofpolysiloxane and the like, fluorine type surfactants such asperfluoroalkylcarboxylate, perfluoroalkylsulfonate, oxyethyleneperfluoroalkyl ether and the like, and biosurfactants such asspicrysporic acid, rhamnolipid, lysolecithin and the like may be used.

[0120] Among the above-exemplified surfactants, nonionic surfactants arepreferable from the viewpoint of the dispersion stability of a pigment.In addition, from the viewpoint of control of the permeability,acetylene glycol, oxyethylene adduct of acetylene glycol,polyoxyethylene alkyl ether and the like are particularly preferable.

[0121] An amount of the surfactant to be added to the ink is preferablyless than 10% by mass, more preferably in the range of 0.01 to 5% bymass, further preferably in the range of 0.01 to 3% by mass. When anaddition amount is more than 10% by mass, the optical concentration andthe storage stability of the ink are deteriorated in some cases.

[0122] Further, in the ink of the invention, for the purpose ofcontrolling the properties, for example, improving the ink dischargingproperty, polyethyleneimine, polyamines, polyvinylpyrrolidone,polyethylene glycol, cellulose derivatives such as ethylcellulose,carboxymethylcellulose and the like, polysaccharides and derivativesthereof, other water-soluble polymers, polymer emulsions such as acrylicpolymer emulsion, polyurethane emulsion and the like, cyclodextrin,macrocyclic amines, dendrimer, crown ethers, urea and derivativesthereof, acetamide and the like can be used.

[0123] In addition, in order to adjust the conductivity or a pH, alkalimetal compounds such as potassium hydroxide, sodium hydroxide, lithiumhydroxide and the like, nitrogen-containing compounds such as ammoniumhydroxide, triethanolamine, diethanolamine, ethanolamine,2-amino-2-methyl-1-propanol and the like, alkaline earth metal compoundssuch as calcium hydroxide and the like, acids such as sulfuric acid,hydrochloric acid, nitric acid and the like, and salts of strong acidsand weak alkalis such as ammonium sulfate can be used.

[0124] Besides, if necessary, a pH buffering agent, an antioxidant, amildewproofing agent, a viscosity modifier, an electroconductive agent,an ultraviolet ray absorbing agent and a chelating agent may be added.

[0125] (Inkjet Recording Method)

[0126] Then, the inkjet recording method of the invention will beexplained. The inkjet recording method of the invention is notparticularly limited as far as the method is a method for printing on arecording medium using the ink of the invention, but specifically, thefollowing method is preferable.

[0127] In the inkjet recording method of the invention, it is suitableto use a recording medium containing a multivalent metal salt, anorganic cationic substance or an organic anionic substance as arecording medium used in printing.

[0128] By using such the recording medium in combination with the ink ofthe invention, it becomes possible to rapidly aggregate a coloringmaterial such as a pigment and the like contained in the ink on therecording medium, and it becomes possible to further improve the imagequality such as the optical concentration, the bleeding and the like.

[0129] The number of particles having a particle diameter of 5 μm orlarger contained in the ink that is added dropwise to the surface of theaforementioned recording medium containing a multivalent metal salt, anorganic cationic substance or an organic anionic substance is preferably1.0×10²/μL or more, more preferably 5.0×10²/μL or more, furtherpreferably 1.0×10³/μL or more.

[0130] When the number of particles having a particle diameter of 5 μmor larger contained in the ink that is added dropwise to the surface ofthe recording medium is less than 1.0×10²/μL, the optical concentrationand the bleeding are insufficient in some cases.

[0131] Measurement of the number of particles having a particle diameterof 5 μm or larger contained in the ink that is added dropwise to thesurface of a recording medium containing a multivalent metal salt, anorganic cationic substance or an organic anionic substance (hereinafter,these substances are abbreviated as “ink flocculant” in some cases) isperformed by adding 0.1 ml of an ink dropwise to the surface of arecording medium containing an ink flocculant, collecting 2 μL of theink present on the surface of the recording medium within five secondsafter adding dropwise, and measuring the number using Accusizer ™770Optical Particle Sizer (manufactured by Particle Sizing Systems).

[0132] At measurement, as a setting parameter to be inputted in theaforementioned measuring apparatus, a density of a pigment is used as adensity of a particle to be dispersed in the aforementioned mixedsolution. The density of a pigment can be obtained by measuring a powderobtained by heating and drying a pigment dispersion using a gravimeteror a pycnometer.

[0133] Alternatively, as the inkjet recording method of the invention,it is suitable to apply a liquid composition containing a multivalentmetal salt, an organic cationic substance or an organic anionicsubstance to the surface of a recording medium and, thereafter, printingon a region of the surface of the recording medium to which the liquidcomposition has been applied, using the ink of the invention. As themultivalent metal salt, the organic cationic substance and the organicanionic substance contained in the liquid composition, substancessimilar to those (ink flocculant) contained in the surface of theabove-explained recording medium can be used.

[0134] By printing with the ink of the invention after the liquidcomposition is applied to the recording medium in this way, it becomespossible to rapidly aggregate a coloring material such as a pigment andthe like contained in the ink on the recording medium, and becomespossible to further improve the image quality such as the opticalconcentration, the bleeding and the like.

[0135] The number of particles having a particle diameter of 5 μm orlarger contained in a mixed solution obtained by mixing theaforementioned liquid composition containing a multivalent metal salt,an organic cationic substance or an organic anionic substance, that is,an ink flocculant, with the ink of the invention is preferably1.0×10³/μL or more, more preferably 5.0×10³/μL or more, furtherpreferably 1.0×10⁴/μL or more.

[0136] When the number of particles having a particle diameter of 5 μmor larger contained in the ink added dropwise to the surface of therecording medium is less than 1.0×10³/μL, the optical concentration andthe bleeding are insufficient in some cases.

[0137] Measurement of the number of particles having a particle diameterof 5 μm or larger contained in a mixed solution obtained by mixing aliquid composition containing an ink flocculant and the ink of theinvention, is performed by mixing an ink and a liquid composition at aratio by mass of 1:1, collecting 2 /μL of the mixed solution whilestirring the solution and, thereafter, measuring the number within tenminutes from the mixing, using Accusizer ™770 Optical Particle Sizer(manufactured by Particle Sizing Systems). Other conditions such assetting of an apparatus at measurement and the like are the same asdescribed above.

[0138] A liquid composition used in the inkjet recording method of theinvention is not particularly limited as far as the composition containsan ink flocculant, but a surfactant may be added to the composition. Asthis surfactant, the same surfactants as those that can be added to theink can be used.

[0139] The surface tension of the liquid composition is preferably atleast 20 mN/m and less than 45 mN/m, more preferably at least 25 mN/mand less than 50 mN/m, further preferably at least 27.5 mN/m and lessthan 37.5 mN/m.

[0140] When the surface tension becomes less than 20 mN/m, the liquidcomposition permeates into the recording medium faster, and colorunevenness due to a printing sequence, the optical concentration and thebleeding are deteriorated in some cases. Conversely, when the surfacetension exceeds 40 mN/m, the liquid composition permeates into therecording medium slower, and unevenness in solid part concentrationoccurs in some cases.

[0141] The viscosity of the liquid composition is preferably at least1.2 mPa·s and less than 6.0 mPa·s, more preferably at least 1.5 mPa·sand less than 4.5 mPa·s, further preferably at least 1.8 mPa·s and lessthan 4.0 mPa·s.

[0142] When the viscosity of the liquid composition is larger than 6.0mPa·s, the discharging property from a nozzle is reduced, and thereliability is reduced in some cases.

[0143] On the other hand, when the viscosity of the liquid compositionis less than 1.2 mPa·s, the sufficient optical concentration can not beobtained. It is contemplated that this is because the permeability ofthe liquid composition into a recording medium becomes great, and, as aresult, a pigment permeates into the interior of the recording medium.

[0144] Then, the details of an ink flocculant contained on the surfaceof the recording medium or in the liquid composition, that is, amultivalent metal salt, an organic cationic substance and an organicanionic substance are explained.

[0145] Examples of the multivalent metal salt contained on the surfaceof the recording medium or the multivalent metal salt which can be addedto the liquid composition include salts of multivalent metal ions suchas aluminium ion, barium ion, calcium ion, copper ion, iron ion,magnesium ion, manganese ion, nickel ion, tin ion, titanium ion, zincion and the like, with hydrochloric acid, hydrobromic acid, hydriodicacid, sulfuric acid, nitric acid, phosphoric acid and thiocyanic acid,and organic carboxylic acids such as acetic acid, oxalic acid, lacticacid, fumaric acid, citric acid, salicylic acid, benzoic acid and thelike, and organic sulfonic acids.

[0146] Specific examples include salts of multivalent metals such asaluminium chloride, aluminium bromide, aluminium sulfate, aluminiumnitrate, sodium aluminium sulfate, potassium aluminium sulfate,aluminium acetate, barium chloride, barium bromide, barium iodide,barium oxide, barium nitrate, barium thiocyanate, calcium chloride,calcium bromide, calcium iodide, calcium nitrite, calcium nitrate,dihydrogencalcium phosphate, potassium thiocyante, calcium benzoate,calcium acetate, calcium salicylate, calcium tartrate, calcium lactate,calcium fumarate, calcium citrate, copper chloride, copper bromide,copper sulfate, copper nitrate, copper acetate, iron chloride, ironbromide, iron iodide, iron sulfate, iron nitrate, iron oxalate, ironlactate, iron fumarate, iron citrate, magnesium chloride, magnesiumbromide, magnesium iodide, magnesium sulfate, magnesium nitrate,magnesium acetate, magnesium lactate, manganese chloride, manganesesulfate, manganese nitrate, dihydrogenmanganase phosphate, manganeseacetate, manganese salicylate, manganese benzoate, manganese lactate,nickel chloride, nickel bromide, nickel sulfate, nickel nitrate, nickelacetate, tin sulfate, titanium chloride, zinc chloride, zinc bromide,zinc sulfate, zinc nitrate, zinc thiocyanate, zinc acetate and the like.

[0147] Subsequently, examples of the organic cationic substance includeprimary, secondary, tertiary and quaternary amines and salt thereof.Specific examples include tetraalkylammonium salt, alkylamine salt,benzalkonium salt, alkylpyridium salt, imidazolium salt, polyamine andthe like, for example, isopropylamine, isobutylamine, t-butylamine,2-ethylhexylamine, nonylamine, dipropylamine, diethylamine,trimethylamine, triethylamine, dimethylpropylamine, ethylenediamine,propylenediamine, hexamethylenediamine, diethylenetriamine,tetraethylenepentamine, diethanolamine, diethylethanolaimne,triethanolamine, tetramethylammonium chloride, tetraethylammoniumbromide, dihydroxyethylstearylamine,2-heptadecenyl-hydroxyethylimidazoline, lauryldimethylbenzylammoniumchloride, cetylpyridinium chloride, stearamidomethylpyridinium chloride,diallyldimethylammonium chloride polymer, diallylamine polymer,monoallylamine polymer and the like.

[0148] Preferable examples include aluminium sulfate, calcium chloride,calcium nitrate, calcium acetate, magnesium chloride, magnesium nitrate,magnesium sulfate, magnesium acetate, tin sulfate, zinc chloride, zincnitrate, zinc sulfate, zinc acetate, aluminium nitrate, monoallylaminepolymer, diallylamine polymer, diallyldimethylammonium chloride polymerand the like.

[0149] In addition, as the organic anionic substance, carboxylic acid,sulfonic acid and phosphoric acid can be used, and examples thereofinclude acetic acid, oxalic acid, lactic acid, fumaric acid, citricacid, salicylic acid, benzoic acid, benzenesulfonic acid,toluenesulfonic acid, hydroxybenzenesulfonic acid, and triphenylphosphate and the like.

[0150] The inkjet recording method of the invention is not particularlylimited as far as the method is the known inkjet system, but printingcan be performed on the surface of a recording medium using a thermalinkjet system and a piezo-inkjet system.

[0151] In the inkjet recording method of the invention, an amount of adroplet (impartation amount) upon impartation of an ink to the surfaceof a recording medium is preferably 25 ng or less, more preferably 0.5ng to 20 ng, further preferably 2.5 ng to 18 ng, per liquid droplet.

[0152] When an amount of a drop exceeds 25 ng per liquid droplet, thebleeding occurs to a high degree in some cases, and a drying timebecomes long in some cases. This is contemplated as follows: Forexample, when a liquid composition having the function of aggregatingink is present on the surface of a recording medium, an ink isaggregated on a plane on which the ink and the recording medium arecontacted, permeation of the ink is inhibited and, thereupon, when anamount of a droplet of an ink is large, the ink is spread in a papersurface direction.

[0153] From the viewpoint of the effect of improving the bleeding, it ismore preferable to use a thermal inkjet system. Although what causes theadvantage of a thermal inkjet system has not been made clear, it iscontemplated as follows: When an ink is discharged from a nozzle, theviscosity of the heated ink becomes low, but since a temperature of theink is lowered when imparted to the surface of a recording medium, theviscosity is dramatically increased. For this reason, it is contemplatedthat this system has the effect of improving the bleeding.

[0154] The ink and the inkjet recording method of the invention asexplained above can be used in an inkjet recording apparatus with aheater for controlling drying of an ink mounted thereon, or in an inkjetrecording apparatus which is provided with an intermediate transferringmechanism in which an ink is transferred onto a recording medium such aspaper after printed on an intermediate body, as well as in a normalinkjet recording apparatus.

EXAMPLES

[0155] The present invention will be explained more specifically by wayof Examples below. However, the invention is not limited by thefollowing Examples.

[0156] (Ink)

[0157] Inks used in Examples are prepared utilizing the followingpigment treating method 1, pigment treating method 2 or ink preparingmethod.

[0158] -Pigment Treating Method 1-

[0159] A pigment is subjected to surface oxidizing treatment with sodiumhypochlorite, and subjected to desalting treatment. The thus obtainedsurface-treated pigment is added to ion-exchanged water so that thepigment concentration becomes 20% by mass, a pH is adjusted to 7.5, anddispersing is performed using an ultrasound homogenizer. This dispersionis subjected to centrifugation treatment (8000 rpm×30 min) with acentrifuging apparatus, to remove the residue part (20% by mass relativeto a total amount).

[0160] -Pigment Treating Method 2-

[0161] 3 Parts by mass of a Styren-Sodium acrylic acid copolymer andion-exchanged water are added to 30 parts by mass of a pigment (BlackPearls L, manufactured by Cabot Corporation), making a total amount of300 parts by mass. This mixed solution is dispersed with an ultrasoundhomogenizer, and this dispersion is subjected to centrifugationtreatment (8000 rpm×30 min) with a centrifugation apparatus to remove100 parts by mass of the residue part. Subsequently, filtration isperformed using a 2 μm filter to obtain a desired pigment dispersion.

[0162] -Ink Preparing Method-

[0163] Appropriate amounts of a water-soluble organic solvent, asurfactant, ion-exchanged water and the like are added to an appropriateamount of the pigment dispersion, and an ink is prepared so thatpredetermined amounts of respective materials are contained. This ismixed, stirred and subjected to a 2 μm filter to obtain a desired ink.

[0164] <Ink-1>

[0165] According to the aforementioned ink preparing method, an inkcontaining the following composition (Ink-1) is prepared.

[0166] Cabojet-300 (manufactured by Cabot Corporation): 4% by mass

[0167] Diethylene glycol (SP=15.0): 8% by mass

[0168] Triethylene glycol (SP=13.6): 4% by mass

[0169] N-methyl-2-pyrrolidone (SP=10.8): 2% by mass

[0170] Urea: 5% by mass

[0171] Acetylene glycol ethylene oxide adduct: 1% by mass

[0172] Ion-exchanged water: 76% by mass

[0173] The viscosity of the ink is 2.3 mPa·s, and the surface tension ofthe ink is 32 mN/m.

[0174] <Ink-2>

[0175] According to the aforementioned ink preparing method, an inkcontaining the following composition (Ink-2) is prepared.

[0176] IJX253 (Cabot Corporation): 5% by mass

[0177] n-Butyl methacrylate-metacrylic acid-sodium methacrylate

[0178] copolymer: 0.15% by mass

[0179] Glycerin (SP=20.0): 10% by mass

[0180] Tripropylene glycol (SP=12.1): 5% by mass

[0181] Butylcarbitol (SP=10.5): 3% by mass

[0182] Urea: 4% by mass

[0183] Polyoxyethylene2-ethyl hexyl ether: 1% by mass

[0184] Ion-exchanged water: 71.85% by mass

[0185] The viscosity of this ink is 2.8 mPa·s, and the surface tensionof the ink is 33 mN/m.

[0186] <Ink-3>

[0187] According to the pigment treating method 1, a pigment is treatedand, subsequently, according to the aforementioned ink preparing method,an ink containing the following composition (Ink-3) is prepared.

[0188] C.I. Pigment Red 122: 4% by mass

[0189] Styrene-methacrylic acid-sodium methacrylate copolymer: 0.1% bymass

[0190] Thiodiethanol (SP=15.2): 8% by mass

[0191] Tetraethylene glycol (SP=12.8): 8% by mass

[0192] Ethylene oxide adduct of propylene glycol (SP=10.9): 4% by mass

[0193] Urea: 5% by mass

[0194] Ethylene oxide adduct of acetylene glycol: 0.8% by mass

[0195] Ion-exchanged water: 70.1% by mass

[0196] The viscosity of this ink is 2.5 mPa·s, and the surface tensionof the ink is 31 mN/m.

[0197] <Ink-4>

[0198] According to the pigment treating method 2, a pigment is treatedand, subsequently, according to the aforementioned ink preparing method,an ink containing the following composition (Ink-4) is prepared.

[0199] Black Pearls L (manufactured by Cabot Corporation): 4% by mass

[0200] Styren-Sodium acrylic acid copolymer: 0.4% by mass

[0201] Diethylene glycol (SP=15.0): 10% by mass

[0202] Triethylene glycol (SP=13.6): 5% by mass

[0203] Ethylene oxide adduct of diglycerin (SP=11.6): 3.5% by mass

[0204] Urea: 6% by mass

[0205] Ethylene oxide adduct of acetylene glycol: 0.25% by mass

[0206] Ion-exchanged water: 70.85% by mass

[0207] The viscosity of this ink is 2.5 mPa·s, and the surface tensionof the ink is 31 mN/m.

[0208] <Ink-5>

[0209] According to the pigment treating method 1, a pigment is treatedand, subsequently, according to the aforementioned ink preparing method,an ink containing the following composition (Ink-5) is prepared.

[0210] C.I. Pigment Red 122: 4% by mass

[0211] Diethylene glycol (SP=15.0): 15% by mass

[0212] N-methyl-2-pyrrolidone (SP=10.8): 5% by mass

[0213] Urea: 5% by mass

[0214] Ethylene oxide adduct of acetylene glycol: 1% by mass

[0215] Ion-exchanged water: 70% by mass

[0216] The viscosity of this ink is 2.2 mPa·s, and the surface tensionof the ink is 32 mN/m.

[0217] <Ink-6>

[0218] According to the pigment treating method 1, a pigment is treatedand, subsequently, according to the aforementioned ink preparing method,an ink containing the following composition (Ink-6) is prepared.

[0219] C.I. Pigment Red 122: 4% by mass

[0220] Diethylene glycol (SP=15.0): 4% by mass

[0221] Triethylene glycol (SP=13.6): 8% by mass

[0222] N-methyl-2-pyrrolidone (SP=10.8): 4% by mass

[0223] Urea: 5% by mass

[0224] Ethylene oxide adduct of acetylene glycol: 1% by mass

[0225] Ion-exchanged water: 74% by mass

[0226] The viscosity of this ink is 2.5 mPa·s, and the surface tensionof the ink is 32 mN/m.

[0227] <Ink-7>

[0228] According to the pigment treating method 1, a pigment is treatedand, subsequently, according to the aforementioned ink preparing method,an ink containing the following composition (Ink-7) is prepared.

[0229] C.I. Pigment Red 122: 4% by mass

[0230] Diethylene glycol (SP=15.0): 8% by mass

[0231] Triethylene glycol (SP=13.6): 4% by mass

[0232] N-methyl-2-pyrrolidone (SP=10.8): 0.5% by mass

[0233] Urea: 5% by mass

[0234] Ethylene oxide adduct of acetylene glycol: 1% by mass

[0235] Ion-exchanged water: 77.5% by mass

[0236] The viscosity of this ink is 2.1 mPa·s, and the surface tensionof the ink is 32 mN/m.

[0237] <Ink-8>

[0238] According to the pigment treating method 1, a pigment is treatedand, subsequently, according to the aforementioned ink preparing method,an ink containing the following composition (Ink-8) is prepared.

[0239] Black Pearls L (manufactured by Cabot Corporation): 4% by mass

[0240] Diethylene glycol (SP=15.0): 8% by mass

[0241] Triethylene glycol (SP=13.6): 4% by mass

[0242] N-methyl-2-pyrrolidone (SP=10.8): 3.2% by mass

[0243] Urea: 5% by mass

[0244] Acetylene glycol ethylene oxide adduct: 1% by mass

[0245] Ion-exchanged water: 74.8% by mass

[0246] The viscosity of this ink is 2.2 mPa·s, and the surface tensionof the ink is 32 mN/m.

[0247] <Ink-9>

[0248] According to the pigment treating method 2, a pigment is treatedand, subsequently, according to the aforementioned ink preparing method,an ink containing the following composition (Ink-9) is prepared.

[0249] Black Pearls L (manufactured by Cabot Corporation): 4% by mass

[0250] Styren-Sodium acrylic acid copolymer: 0.4% by mass

[0251] Diethylene glycol (SP=15.0): 8% by mass

[0252] Ethylene glycol (SP=17.8): 2% by mass

[0253] Triethylene glycol (SP=13.6): 3.5% by mass

[0254] Tetraethylene glycol (SP=12.8): 0.5% by mass

[0255] 1,2-Hexanediol (SP=13.4): 1% by mass Ethylene oxide adduct ofdiglycerine (SP=11.6): 1% by mass Butylcarbitol (SP=10.5): 0.5% by mass

[0256] Urea: 6% by mass

[0257] Ethylene oxide adduct of acetylene glycol: 0.25% by mass

[0258] Ion-exchanged water: 72.85% by mass

[0259] The viscosity of this ink is 2.3 mPa·s, and the surface tensionof the ink is 31 mN/m. A solubility parameter SP₁ of a firstwater-soluble solvent group contained in the ink obtained by the alreadydescribed equation (3) is 13.5.

[0260] <Ink-10>

[0261] According to the pigment treating method 1, a pigment is treatedand, subsequently, according to the aforementioned ink preparing method,an ink containing the following composition (Ink-10) is prepared.

[0262] Black Pearls L (manufactured by Cabot Corporation): 4% by mass

[0263] Diethylene glycol (SP=15.0): 8% by mass

[0264] Triethylene glycol (SP=13.6): 4% by mass

[0265] N-methyl-2-pyrrolidone (SP=10.8): 0.8% by mass

[0266] Urea: 5% by mass

[0267] Ethylene oxide adduct of acetylene glycol: 1% by mass

[0268] Ion-exchanged water: 77.2% by mass

[0269] The viscosity of this ink is 2.2 mPa·s, and the surface tensionof the ink is 32 mN/m.

[0270] <Ink-11>

[0271] According to the pigment treating method 1, a pigment is treatedand, subsequently, according to the aforementioned ink preparing method,an ink containing the following composition (Ink-11) is prepared.

[0272] Black Pearls L (manufactured by Cabot Corporation): 4% by mass

[0273] Diethylene glycol (SP=15.0): 3.6% by mass

[0274] Triethylene glycol (SP=13.6): 4% by mass

[0275] N-methyl-2-pyrrolidone (SP=10.8): 2% by mass

[0276] Urea: 5% by mass

[0277] Ethylene oxide adduct of acetylene glycol: 1% by mass

[0278] Ion-exchanged water: 80.4% by mass

[0279] The viscosity of this ink is 2.0 mPa·s, and the surface tensionof the ink is 31 mN/m.

[0280] <TL1>

[0281] A liquid composition (TL-1) is obtained by mixing the followingcomposition.

[0282] Diethylene glycol: 20% by mass

[0283] Magnesium nitrate: 5% by mass

[0284] Urea: 5% by mass

[0285] Polyoxyethylene stearyl ether (the number of repeating units ofethylene oxide=12): 0.1% by mass

[0286] Water: 69.9% by mass

[0287] The viscosity of this liquid composition is 2.1 mPa·s and thesurface tension is 31 mN/m.

[0288] (Assessment Method)

[0289] Printing is performed only with ink, or with ink and a liquidcomposition, on a FX-P paper (manufactured by Fuji Xerox Co., Ltd.)using a 800 dpi trial printing head with 256 nozzles, and variousassessments are performed. Unless specified otherwise, printing isperformed under the general environment (temperature: 23±0.5° C.,humidity: 55±5% R.H), and assessment was performed on samples which hasbeen allowed to stand under the general environment for 24 hours afterprinting.

[0290] Regarding Examples 1 to 5 using Ink-1 to 4, and 9, andComparative Examples 1 to 6 using Ink-5 to 8, 10, and 11, assessment ofthe ink alone is performed. The results are shown in Table 1.

[0291] On the other hand, regarding Examples 6 and 7 using Ink-2 and 3and TL1 in combination, a liquid composition is imparted to a recordingmedium, then, the ink is imparted. Assessment is performed using suchrecording method. The results are shown in Table 2. TABLE 1 ζ Long-Long- poten- term term Optical Water SP₂ − SP₁ − tial η γ ejectionstorage concen- , resis- Ink SP₁ SP₃ W₂/W₁ W₃/W₁ (mV) (mPa · s) (mN/m)stability stability tration Bleeding tance Example 1 Ink-1 1.4 2.8 2 0.5−32 2.3 32 ◯ ◯ ◯ ◯ ◯ Example 2 Ink-2 7.9 1.6 2 0.6 −50 2.8 33 ◯ ◯ ◯ ◯ ΔExample 3 Ink-3 2.4 1.9 1 0.5 −11 2.5 31 ◯ ◯ ◯ ◯ ◯ Example 4 Ink-4 1.42.0 2 0.7 −44 2.5 31 ◯ ◯ ◯ ◯ ◯ Example 5 Ink-9 1.4 2.0 2 0.3 −38 2.3 31◯ ◯ ◯ ◯ ◯ Comparative Ink-5 — — — — −15 2.2 32 X ◯ ◯ ◯ ◯ Example 1Comparative Ink-6 1.4 2.8 0.5 0.5 −11 2.5 32 X X ◯ ◯ ◯ Example 2Comparative Ink-7 1.4 2.8 2 0.125 −9 2.1 32 ◯ X ◯ ◯ ◯ Example 3Comparative Ink-8 1.4 2.8 2 0.8 −63 2.2 32 X ◯ X X X Example 4Comparative Ink-10 1.4 2.8 2 0.2 −65 2.2 32 ◯ X X X X Example 5Comparative Ink-11 1.4 2.8 0.9 0.5 −66 2.0 31 ◯ X X X X Example 6

[0292] TABLE 2 Long- Long- Ink and ζ term term Optical Water liquid SP₂− SP₁ − potential η γ ejection storage concen- resis- composition SP₁SP₃ W₂/W₁ W₃/W₁ (mV) (mPa · s) (mN/m) stability stability trationBleeding tance Example 6 Ink-2 7.9 1.6 2 0.6 −50 2.8 33 ◯ ◯ ◯ ◯ ◯ TL-1 —— — — — 2.1 31 Example 7 Ink-3 2.4 1.9 1 0.5 −11 2.5 31 ◯ ◯ ◯ ◯ ◯ TL-1 —— — — — 2.1 31

[0293] In Table 1 and Table 2, “SP₂” means a solubility parameter of asecond water-soluble solvent group (when two or more kinds ofwater-soluble solvents are contained, a value of a water-soluble solventhaving the lowest solubility parameter), “SP₃” means a solubilityparameter of a third water-soluble solvent group (when two or more kindsof water-soluble solvents are contained, a value of a water-solublesolvent having the highest solubility parameter), “η” means theviscosity and “λ” means the surface tension. In addition, an assessingmethod and assessment criteria of various assessment items (long-termejection stability, long-term storage stability, optical concentration,bleeding and water resistance) shown in the above Table 1 and Table 2are as explained below.

[0294] <Long Term Ejection Stability>

[0295] An ink is continuously printed at 5×10⁷ pulse, and Drop speedsare measured before and after printing. A rate of Drop speeds before andafter continuous printing (ADS value (%)) is calculated according to thefollowing equation (5), and assessment is performed based on thefollowing assessment criteria.

ΔDS=100×(Drop speed after continuous printing÷Drop speed beforecontinuous printing)  Equation (5)

[0296] -Assessment Criteria-

[0297] ◯ . . . ΔDS is 90% or more.

[0298] Δ . . . ΔDS is 80% or more.

[0299] X . . . ΔDS is less than 80%.

[0300] <Long-term Storage Stability>

[0301] Ink is allowed to stand at a temperature of 60° C. for 1000hours, and the number of particles having a particle diameter of 0.5 μmor larger contained in an ink before and after standing is measured.Then, a ratio of the numbers of particles before and after standing (ΔPSvalue (%)) is calculated according to the following equation (6) andassessment is performed based on the following assessment criteria.

[0302] Measurement of the number of particles having a particle diameterof 0.5 μm or larger is performed using Accusizer ™770 Optical ParticleSizer (manufactured by Particle Sizing Systems) by the same measuringmethod as that described above.

ΔPS=100×(the number of particles having a particle diameter of 0.5 μm orlarger contained in the ink after standing÷the number of particleshaving a particle diameter of 0.5 μm or larger contained in an inkbefore standing)  Equation 6

[0303] -Assessment Criteria-

[0304] ◯ . . . ΔPS is less than 110%.

[0305] Δ . . . ΔPS is less than 150%.

[0306] X . . . ΔPS is 150% or more.

[0307] <Optical Concentration>

[0308] A 100% coverage pattern is printed, the optical concentration ismeasured using X-rite 404 (manufactured by X-rite Incorporated.), andranking is performed based on the following assessment criteria.

[0309] -Assessment Criteria (in the Case of Using Black Ink)-

[0310] ◯ . . . Optical concentration is at least 1.4.

[0311] Δ . . . Optical concentration is at least 1.3 and less than 1.4.

[0312] X . . . Optical concentration is less than 1.3.

[0313] -Assessment Criteria (in the Case of Using Color Ink)-

[0314] ◯ . . . Optical concentration is at least 1.1.

[0315] Δ . . . Optical concentration is at least 1.0 and less than 1.1.

[0316] X . . . Optical concentration is less than 1.0.

[0317] <Bleeding>

[0318] A fine line pattern was printed, an extent of the bleeding of aprinted part is compared with a boundary sample, and sensory assessmentis performed according to the following assessment criteria.

[0319] -Assessment Criteria-

[0320] ◯ . . . Slight bleeding.

[0321] Δ . . . Bleeding occured at an acceptable level.

[0322] X . . . Bleeding was severe and is outside an acceptable range.

[0323] <Water Resistance>

[0324] A sample on which a solid pattern has been printed is immersed inwater for 5 minutes, and assessment is performed using the followingassessment criteria based on the rate of the optical concentrationsbefore and after immersion (ΔOD value (%) calculated according to thefollowing equation (7).

ΔOD value=100×(optical concentration after immersion÷opticalconcentration before immersion)  Equation (7)

[0325] -Assessment Criteria-

[0326] ◯ . . . ΔOD value is at least 90%.

[0327] Δ . . . ΔOD value is at least 80%.

[0328] X ΔOD value is less than 80%.

[0329] As explained above, the inkjet ink and the inkjet recordingmethod recited in the invention are excellent in the long-term ejectionstability, the long-term storage stability, the optical concentration,the bleeding and the water resistance.

What is claimed is:
 1. An inkjet ink comprising at least a pigment, awater-soluble solvent and water, the water-soluble solvent comprising: afirst water-soluble solvent group containing at least one kind ofwater-soluble solvent represented by the following general formula (I);a second water-soluble solvent group containing at least one kind ofwater-soluble solvent having a solubility parameter which is at least 1greater than a solubility parameter SP₁ of the at least one kind ofwater-soluble solvent represented by general formula (I), and; a thirdwater-soluble solvent group containing at least one kind ofwater-soluble solvent having a solubility parameter which is at least 1less than the solubility parameter SP₁; wherein respective contents (%by mass) of the water-soluble solvent groups contained in the inkjet inksatisfy the following equation (1) and the following equation (2):HO—(CHR—CH₂—O—)_(n)—H  General formula (I)W ₂ /W ₁≧1.0  Equation(1)0.25≦W ₃ /W ₁≦0.75  Equation (2) wherein in general formula (I),equation (1) and equation (2), n represents an integer of 3 to 6; Rrepresents hydrogen or a methyl group; W₁ represents a content (% bymass) of the first water-soluble solvent group contained in the inkjetink; W₂ represents a content (% by mass) of the second water-solublesolvent group contained in the inkjet ink; and W₃ represent a content (%by mass) of the third water-soluble solvent group contained in theinkjet ink.
 2. An inkjet ink according to claim 1, wherein the firstwater-soluble organic solvent group further contains a water-solublesolvent other than the at least one kind of water-soluble solventrepresented by general formula (I), having a solubility parameter suchthat an absolute value of a difference thereof from the solubilityparameter SP₁ is less than
 1. 3. An inkjet ink according to claim 1,wherein the pigment is self-dispersible in water.
 4. An inkjet inkaccording to claim 3, further containing a polymer anion or a polymercation.
 5. An inkjet ink according to claim 1, further comprising apolymer dispersant, wherein the pigment is dispersed by the polymerdispersant.
 6. An inkjet ink according to claim 2, wherein an absolutevalue of a ζ potential of the inkjet ink is in the range of 3 mV to 60mV.
 7. An inkjet ink according to claim 1, wherein a surface tension ofthe inkjet ink is at least 20 Nm/m and less than 45 mN/m.
 8. An inkjetink according to claim 1, wherein a viscosity of the inkjet ink is atleast 1.2 mPa·s and less than 6.0 mPa·s.
 9. An inkjet recording methodof printing on a recording medium containing a multivalent metal salt,an organic cationic substance or an organic anionic substance, using aninkjet ink containing at least a pigment, a water-soluble solvent andwater, wherein: the water-soluble solvent contains, a firstwater-soluble solvent group containing at least one kind ofwater-soluble solvent represented by the following general formula (I),a second water-soluble solvent group containing at least one kind ofwater-soluble solvent having a solubility parameter which is at least 1greater than a solubility parameter SP₁ of the at least one kind ofwater-soluble solvent represented by general formula (I), and a thirdwater-soluble solvent group containing at least one kind ofwater-soluble solvent having a solubility parameter which is at least 1less than the solubility parameter SP₁; and respective contents (% bymass) of the water-soluble solvent groups contained in the inkjet inksatisfy the following equation (1) and the following equation (2):HO—(CHR—CH₂—O—)_(n)—H  General formula (I)W ₂ /W ₁ ≧1.0  Equation(1)0.25≦W ₃ /W ₁≦0.75  Equation (2) wherein in general formula (I),equation (1) and equation (2), n represents an integer of 3 to 6; Rrepresents hydrogen or a methyl group; W₁ represents a content (% bymass) of the first water-soluble solvent group contained in the inkjetink; W₂ represents a content (% by mass) of the second water-solublesolvent group contained in the inkjet ink; and W₃ represents a content(% by mass) of the third water-soluble solvent group contained in theinkjet ink.
 10. An inkjet recording method according to claim 9, whereinthe first water-soluble organic solvent group further contains awater-soluble solvent other than the at least one kind of water-solublesolvent represented by the general formula (I), having a solubilityparameter such that an absolute value of a difference thereof from thesolubility parameter SP₁ is less than
 1. 11. An inkjet recording methodaccording to claim 9, wherein a number of particles having particlediameters of 5 μm or larger contained in the inkjet ink which is addeddropwise to a surface of the recording medium is 1×10²/μl or more. 12.An inkjet recording method in which a liquid composition containing amultivalent metal salt, an organic cationic substance or an organicanionic substance is imparted to a surface of a recording medium and,thereafter, printing is conducted on a region of the surface of therecording medium to which the liquid composition has been imparted,using an inkjet ink containing at least a pigment, a water-solublesolvent and water, wherein: the water-soluble solvent contains a firstwater-soluble solvent group containing at least one kind ofwater-soluble solvent represented by the following general formula (I),a second water-soluble solvent group containing at least one kind ofwater-soluble solvent having a solubility parameter which is at least 1greater than a solubility parameter SP₁ of the at least one kind ofwater-soluble solvent represented by general formula (I), and a thirdwater-soluble solvent group containing at least one kind ofwater-soluble solvent having a solubility parameter which is at least 1less than the solubility parameter SP₁; and respective contents (% bymass) of the water-soluble solvent groups contained in the inkjet inksatisfy the following equation (1) and the following equation (2):HO—(CHR—CH₂—O—)_(n)—H  General formula (I)W ₂ /W ₁≧1.0  Equation(1)0.25≦W ₃ /W ₁≦0.75  Equation (2) wherein in general formula (I),equation (1) and equation (2), n represents an integer of 3 to 6; Rrepresents hydrogen or a methyl group; W₁ represents a content (% bymass) of the first water-soluble solvent group contained in the inkjetink; W₂ represents a content (% by mass) of the second water-solublesolvent group contained in the inkjet ink; and W₃ represents a content(% by mass) of the third water-soluble solvent group contained in theinkjet ink.
 13. An inkjet recording method according to claim 12,wherein the first water-soluble organic solvent group further contains awater-soluble solvent other than the at least one kind of water-solublesolvent represented by the general formula (I), having a solubilityparameter such that an absolute value of a difference thereof from thesolubility parameter SP₁ is less than
 1. 14. An inkjet recording methodaccording to claim 12, wherein a number of particles having particlediameters of 5 μm or larger in a mixed solution of the inkjet ink andthe liquid composition is 1.0×10³/μl or more.
 15. An inkjet recordingmethod of printing on a recording medium by a thermal inkjet system or apiezo-inkjet system using an inkjet ink containing at least a pigment, awater-soluble solvent and water, wherein: the water-soluble solventcontains a first water-soluble solvent group containing at least onekind of water-soluble solvent represented by the following generalformula (I), a second water-soluble solvent group containing at leastone kind of water-soluble solvent having a solubility parameter which isat least 1 greater than a solubility parameter SP₁ of the at least onekind of water-soluble solvent represented by general formula (I), and athird water-soluble solvent group containing at least one kind ofwater-soluble solvent having a solubility parameter which is at least 1less than the solubility parameter SP₁; and respective contents (% bymass) of the water-soluble solvent groups contained in the inkjet inksatisfy the following equation (1) and the following equation (2):HO—(CHR—CH₂—O—)_(n)—H  General formula (I)W ₂ /W ₁≧1.0  Equation(1)0.25≦W ₃ /W ₁≦0.75  Equation (2) wherein in general formula (I),equation (1) and equation (2), n represents an integer of 3 to 6; Rrepresents hydrogen or a methyl group; W₁ represents a content (% bymass) of the first water-soluble solvent group contained in the inkjetink; W₂ represents a content (% by mass) of the second water-solublesolvent group contained in the inkjet ink; and W₃ represents a content(% by mass) of the third water-soluble solvent group contained in theinkjet ink.
 16. An inkjet recording method according to claim 15,wherein the first water-soluble organic solvent group contains awater-soluble solvent other than the at least one kind of water-solublesolvent represented by general formula (I), having a solubilityparameter such that an absolute value of a difference thereof from thesolubility parameter SP₁ is less than
 1. 17. An inkjet recording methodaccording to claim 15, wherein an amount of the inkjet ink to beimparted to a surface of the recording medium is 25 ng or less per onedroplet.